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See the recommended documentation of this function

# diag

diagonal including or extracting

### Calling Sequence

[y]=diag(vm, [k])

### Arguments

- vm
vector or matrix (full or sparse storage)

- k
integer (default value 0)

- y
vector or matrix

### Description

for `vm`

a (row or column) n-vector
`diag(vm)`

returns a diagonal matrix with entries of
`vm`

along the main diagonal.

`diag(vm,k)`

is a
`(n+abs(k))x(n+abs(k))`

matrix with the entries of
`vm`

along the kth diagonal. `k=0`

is
the main diagonal `k>0`

is for upper diagonals and
`k<0`

for lower diagonals.

For a matrix `vm`

, `diag(vm,k)`

is
the column vector made of entries of the kth diagonal of
`vm`

. `diag(vm)`

is the main diagonal of
`vm`

. `diag(diag(x))`

is a diagonal
matrix.

If `vm`

is a sparse matrix
`diag(vm,k)`

returns a sparse matrix.

To construct a diagonal linear system, use
`sysdiag`

.

Note that `eye(A).*A`

returns a diagonal matrix
made with the diagonal entries of `A`

. This is valid for
any matrix (constant, polynomial, rational, state-space linear
system,...).

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